a) Field of the Invention
The present invention relates to an optical lens system which is to be used in various kinds of optical devices.
b) Description of the Prior Art
It is generally desirable for a zoom optical system to correct aberrations independently in each lens unit so that aberrations are not only corrected in a standard position but also varied little by changing a magnification. To meet this goal, each lens unit is usually composed of a plurality of lens elements.
Recently, compact configuration and lowering of manufacturing cost are desirable for zoom optical devices to be used in various kinds of optical systems. For making a zoom optical system more compact, it is sufficient from the paraxial theory to increase the refractive power of a lens unit which has a vari-focal function, thereby shortening the distance which the lens unit must move to produce a given change of magnification. When a refractive power of the lens unit is increased, however, the lens unit produces greater aberration, to reduce the amount of aberration it is necessary, to compose the lens unit of an increased number of lens elements. When the lens unit is composed of an increased number of lens elements, it is impossible to maintain compact size and low cost. This problem sets limits to compact configuration and reduction in manufacturing costs of zoom optical systems which are composed of homogeneous lens elements.
Zoom optical systems may be made more compact by using radial type gradient index lens elements which have refractive index distributions in media thereof in radial directions from optical axes.
Since a radial type gradient index lens element has a refractive index distribution, it may better correct aberrations than may a homogeneous lens element. Owing to a refractive index of the medium in particular, the radial type gradient index lens element has a characteristic excellent in correction of a Petzval's sum and chromatic aberration.
As conventional examples of optical systems which use radial type gradient index lens elements, there are known lens systems disclosed, for example, as fifth and sixth embodiments of Japanese Patent Kokai Publication No.Sho 61-231517, a second embodiment of Japanese Patent Kokai Publication No. Sho 61-248015, and third embodiment of Japanese Patent Kokai Publication No. Hei 2-79013. However, each of these zoom optical systems has a zoom ratio on the order of 3 and is composed of a larger number of lens elements, or 9 to 13 lens elements.